The present invention relates generally to the measurement of various parameters or characteristics of a web of sheet material as it is being manufactured and, more particularly, to a method and apparatus for measuring the relative positions of two heads which are used to perform the measurement such that errors due to variations in the measurement head positions can be compensated.
A variety of systems are known for measuring characteristics of webs of sheet material as the webs are moving in a first machine direction in a manufacturing process. For example, first and second sensing members are often physically moved or swept in a second cross direction generally perpendicular to movement of the web with the members supported for movement on opposite sides of the web by a support frame. For determining basis weight of a paper web, the members may be a beta radiation source on one side of the web and a beta radiation sensor on the opposite side. For moisture content of a paper web, the members may be a source of infrared radiation on one side and a detector to receive infrared radiation on the opposite side.
For many measurement systems, the positioning between the two sensing members or heads can significantly affect the determination of the characteristic being measured. Such measurement effects are particularly apparent in the basis weight measurement noted above. Further, the positioning between the sensor heads can sometimes vary unpredictably such that cross direction distance variation patterns determined for a given support frame and measurement system cannot generally be relied upon.
For non-magnetic materials, a popular method for determining the positioning or relative motion between two sensor heads such that the corresponding characteristic readings can be appropriately compensated is to pass magnetic energy through the material. For example, in U.S. Pat. No. 4,678,915, magnetic energy is generated on one side of a web of material and sensed on the other side. A transmitter of the magnetic energy is fixedly positioned relative to one of the sensor heads and a primary receiver of the magnetic energy is fixedly positioned relative to the other sensor head such that the magnitude of the received magnetic energy is representative of the spacing between the two heads. Ideally, the transmitter and the receiver are axially aligned with one another. To detect and correct for inaccurate alignment, secondary receivers can be positioned adjacent to the primary receiver. One or more pairs of secondary receivers are positioned adjacent to and on opposite sides of the primary receiver with secondary receiver pairs aligned in either the machine direction or the cross direction or both.
While such known prior art position sensing arrangements improve the determinations of characteristic measurement systems which are sensitive to the relative motion of scanned sensor heads relative to one another, there is a need to provide alternatives which are potentially more accurate and have the capacity to provide additional information to operators of characteristic measurement systems.